Description

Surface mount ‘Pi’/’C’ Filters

‘Pi’ filters incorporate 2 capacitors and an inductor together to make a multi-element filter, giving sharper cut-off and better low frequency performance than straight ‘C’ filters. The Syfer range of SM ‘Pi’ filters use conventional MLCC manufacturing techniques to form the capacitive element of the filter, but the inductive element is created separately either by means of a bead inductor placed over a through conductor pin, or by means of a buried layer surface mount inductor with the signal carried by the buried internal layers. Current carrying capacity is defined by the characteristics of the conductor and is independent to the capacitance of the filter.

Insertion Loss

Insertion Loss figures

Insertion loss plots and figures supplied are typical only and are measured on 50Ω stripline open boards, 0.8mm thick FR4 with gold plated tracks. Solder pads are ‘T’ bar with respect to the track and dimensionally match the recommendations given. The boards are mounted to brass support jigs for mechanical stability and electrical grounding. All measurements are taken using a Vector Network Analyser in a 50Ω system, no load. It is important to recognise that the board material, thickness and layout, the plating finish, the grounding efficiency and the circuit impedances will all have an effect on the actual performance of the filter in operation. The effect of the board makes it difficult to directly compare parts unless the mounting details are defined. It is therefore important to test the filters in circuit to determine the performance level achieved.

Effects of mounting method on Insertion Loss

‘C’ and ‘Pi’ filters are mounted to PCBs and soldered in identical manner to chip capacitors. Solder connections made to each end (signal lines) and each side band (earth track). Whilst SBSG, SBSM and SBSP filters can be mounted conventionally on PCBs, they are also suitable for mounting in a wall or partition on a board. This greatly improves the screening between filter input and output, thereby enhancing the high frequency response. The following insertion loss curves based on actual measurements, show the effect. It can be seen that the filters conventionally mounted exhibit a drop in attenuation at higher frequencies. Shielding methods maintain improved suppression characteristics to 1GHz and above.